1. Field of the Invention
The invention relates to an image scanner, and more particularly to an image scanner reading images such as letters and figures, and converting them into digital data.
2. Description of the Related Art
An image scanner employing a linear image sensor for converting images into electric signals together with a sub-scanner to thereby read two-dimensional images is grouped, with respect to a shape, into a flat bed type where a document is turned over and then a box-shaped reader reads the document, a handy type where an operator sub-scans a document with a reader, and a camera type where a reading unit equipped with a linear image sensor is spaced away from a document to be read.
A camera type image scanner has advantages that a document can be read, if only the document is put below a reading unit, and that it is possible to read a document having a thickness such as a book or a three-dimensional document. As examples of a camera type image scanner are known "Image Scanner DS-3000" commercially available from Chinnon Co. and "Oban Sokusha" commercially available from Casio Corporation.
In a camera type image scanner, a document spaced away from a reading unit is focused by means of a lens, and the thus focused partial images are successively read by a linear image sensor with being varied by sub-scanning operation. The partial images read into the linear image sensor is synthesized into two-dimensional image data.
In one of methods of producing two-dimensional images by sub-scanning operation, a sub-scanning reflecting mirror is located between a lens and a document to be read or between a lens and a linear image sensor. Partial images focused onto the linear image sensor are varied by rotating or moving the sub-scanning reflecting mirror, and then, synthesized into two-dimensional images.
FIG. 1 illustrates one of conventional image scanners. The illustrated image scanner includes a linear image sensor 6 for converting images into electric signals, a lens 5 for focusing images of a document 7 to be read onto the linear image sensor 6, a sub-scanning reflecting mirror 1 for successively directing partial images to the linear image sensor 6 through the lens 5, a sub-scanning device 4 for rotating or moving the sub-scanning reflecting mirror 1, a linear image sensor driving circuit 10 which transmits drive signals to the linear image sensor 6, a processing circuit 11 receiving electric signals from the linear image sensor 11, and an analog/digital converter circuit 12 for converting electric signals received from the linear image sensor 6 through the processing circuit 11 into digital electric signals.
The sub-scanning reflecting mirror 1 is generally designed to be rectangular, as illustrated in FIG. 1. The sub-scanning reflecting mirror 1 has to have a certain size or greater defined by an effective length of the linear image sensor 6, a lens aperture and a focal distance of the lens 5, a distance between the sub-scanning reflecting mirror 1 and the document 7, and an area to be read in the document 7.
Various attempts have been made to make an image scanner smaller in a size. For instance, Japanese Unexamined Patent Publication No. 8-214127 published on Aug. 20, 1996 has suggested a flat bed type image scanner wherein a sensor for sensing a size of a document is situated outside a reading unit to thereby enhance flexibility in an arrangement of parts, ensuring a smaller size in an image scanner.
Japanese Unexamined Patent Publication No. 4-311146 published on Nov. 2, 1992 has suggested the use of a focusing transmittal array for an image sensor in order to have image formation by virtue of lens effect and a light path length. In addition, a half mirror and a reflecting mirror are positioned in an image sensor module so that a light path between a document to be read and the image sensor is in parallel with the document. This structure ensures a thin image sensor module a long light path length, resulting in a deeper depth of field.
Japanese Unexamined Patent Publication No. 3-10564 published on Jan. 18, 1991 has suggested an image scanner including a transparent plate and a reflecting plate. A light reflected at a document to be read is made to reflect at a plurality of times in the transparent plate. According to the Publication, it is possible to make a space required for a reflected light to advance smaller with an optical path length of a reflected light being kept sufficient, which ensures a smaller size of an image scanner.
Japanese Unexamined Patent Publication No. 6-78118 published on Mar. 18, 1994 has suggested an image scanner including a case, and a frame fixed in the case. Parts for constituting an image scanner, such as a laser source, a collimator lens, and a reflecting mirror are all secured to the frame, which ensures the smaller number of parts and a smaller size of an image scanner.
In a camera type image scanner, suppose that a document to be read lies on a desk, a reading unit is located about at the level of operator's eyes. Accordingly, if a reading unit is big, an operator cannot look at a document or smoothly handle the image scanner. Thus, a smaller image scanner is still in need.
In order to make a reading unit of a camera type image scanner smaller in a size, it would be necessary to make a sub-scanning reflecting mirror and a sub-scanning device smaller. As mentioned earlier, in an image scanner having a. sub-scanning reflecting mirror designed to be rotatable or movable, the sub-scanning reflecting mirror is generally rectangular, and has to have a certain area or greater in order to prevent deterioration in brightness. In addition, a sub-scanning device for rotating or moving the reflecting mirror has to be positioned outside the reflecting mirror so as not to intercept an optical path of reflected lights.
Thus, a conventional image scanner having a rectangular sub-scanning reflecting mirror cannot avoid having a width equal to a sum of a width of a sub-scanning reflecting mirror and a width of a sub-scanning device. That is, a conventional image scanner has a problem that it cannot be made smaller than a width of a sub-scanning reflecting mirror.
A sub-scanning reflecting mirror varies a reflection angle to thereby successively focus partial images onto a linear image sensor, and is designed to rotate or move. If a movable portion of the reflecting mirror had a great mass, the reflecting mirror would have a great inertia, which causes a problem that the reflecting mirror cannot be readily controlled. This further causes a problem of deterioration of image qualities.
As having been explained so far, it was quite difficult or almost impossible in a conventional image scanner having a rectangular reflecting mirror to make an image scanner smaller in a size without deterioration of brightness and quality of images.